The present invention relates to a rotating device for continuously filtering a liquid such as a solution containing a precipitate.
In numerous cases, there is in fact a need to filter a slurry, a suspension or a precipitate in order to separate the solid matters from the liquid matters.
To that end, a certain number of filtration devices for effecting such a separation, particularly continuously, have already been proposed, the product to be filtered arriving in the device via an inlet, the liquid and the solid matters, these two products being separated from each other, being collected at the outlet of the device.
For example, filtration devices are known, which comprise a cylindrical, horizontal-axis drum of revolution, rotated by a drive system, said drum having a rectangular longitudinal section with a vertical wall, a cylindrical wall and an axial opening defined by an annular flange extending the cylindrical wall, a plurality of filtering elements each disposed axially, forming at least a part of the cylindrical wall of the drum, said filtering elements each being connected to a process conduit.
Such devices also comprise a system for supplying liquid to be filtered, entering via said opening inside said drum, a system for evacuating the cake (solid matters), a system for evacuating the filtrate passing through said process conduits, a hydraulic supply assembly comprising a pulsed compressed air supply and means forcing the passage of said liquid through the filtering elements, said supply assembly being adapted to communicate with said process conduits by a connecting system allowing an angular distribution of the operations effected by said device, these operations comprising at least the filtration of the liquid and the separation of the cake from said filtering elements by cleaning.
However, such filtration devices present a large number of drawbacks. In particular, the continuous filtration devices of the prior art present a complicated structure which does not allow easy maintenance of this device.
It is an object of the invention to provide a rotating continuous-filtration device which does not present the drawbacks of the prior art and which presents a compact, reliable structure easy to maintain, access to the elements to be checked or changed being simplified.
To that end, the invention relates to a filtration device of the type mentioned hereinbefore, wherein said drive system comprises a drive shaft coaxial and fast with said drum at the level of said vertical wall located on the side of the drum opposite said opening, wherein said means forcing the passage of said liquid through the filtering elements comprise a source of depression for filtration connected to said system for evacuation of the filtrate and placing the interior of said filtering elements under depression with respect to the interior of the drum, wherein said system for evacuating the cake functions by gravity and presents a collecting hopper located inside the drum substantially beneath the uppermost part and connected to a spout extending out of the drum via said opening, wherein said process conduits extend in said vertical wall of the drum and present, on the one hand, a first end opening out in a filtering element and, on the other hand, a second end opening out in the outer vertical face of said vertical wall, and wherein said connecting system comprises a first disc (plate) fast with the drum about the drive shaft and pierced with a plurality of connection openings which are connected one by one to the second ends of said process conduits, and a second fixed disc (counter-plate) coaxial with and adjacent said first disc and provided with angular slots in register with said supply assembly and disposed radially opposite said connection openings, an angular filtration slot being located at least partially in the lower part of the second disc and connected to said source of depression for the filtration of the liquid, and an angular cleaning slot being located substantially in the uppermost angular sector of the second disc and connected to said compressed air supply for the separation of the cake from each filtering element.
It is thus understood that the filtration of the liquid from the interior of the drum towards the outside of the drum via the filtering elements, thanks to a source of depression for filtration placing the interior of the filtering elements under depression with respect to the interior of the drum, makes it possible to produce a reduced filtration circuit guaranteeing the passage of the liquid in the filtering elements. In addition, by using a connecting system with a first disc, or plate, which is mobile in rotation, and a second disc, or counter-plate, which is fixed, an angular distribution of the different functions of the device at the level of a very compact structure is reliably obtained.
In order to improve the facility of maintenance of the filtration device according to the invention, the following advantageous arrangements are provided:
the cylindrical wall of the drum is provided with supports each oriented axially and distributed radially in regularly spaced manner, each filtering element being removably mounted on said supports in order to line the whole cylindrical wall;
each filtering element is mounted to slide axially with respect to the supports and comprises a base, or shoe, defining a flow cavity covered, in the radially internal direction, by a filter gauze, the bottom of the cavity being inclined towards an opening for evacuation connected to the first end of the process conduit;
the drum comprises reversible blocking means for blocking each filtering element with respect to the supports;
the blocking means are adapted to place the filtering elements in radial abutment against the supports in the direction of the horizontal axis.
Furthermore, the liquid supply system is provided to comprise a distributor which allows a substantially homogeneous distribution of the liquid to be filtered over the whole length of the filtering elements, parallel to the axis of the drum.
It will therefore be understood that it is in that case possible to effect maintenance of each filtering element independently of one another, whether for replacement or cleaning purposes.
According to other advantageous arrangements, said supply assembly also comprises:
a source of depression for drying which generates a depression greater than the depression furnished by said source of depression for filtration and which is connected to the system for evacuating the filtrate, said second disc further comprising an angular slot for drying disposed between the angular slot for filtration and the angular slot for cleaning, and connected to said source of depression for drying;
a supply of washing solution for the cake opening out via said opening in the drum at the level of a rinsing nozzle adapted to direct the jet in the direction of at least one filtering element located radially opposite said angular slot for drying;
a supply of washing solution for the filtering elements, connected to an angular slot for washing of the second disc, said angular slot for washing being located between the angular slot for cleaning and the angular slot for filtration, in order to allow, outside the periods of filtration, the cleaning of the filtering elements by the passage of said washing solution from the process conduits through the angular slot for washing in the direction of the corresponding filtering elements, this circulation of liquid being effected in counter-flow with respect to the filtration step.
According to other advantageous configurations of the filtration device in accordance with the present invention, this device also comprises:
a system for applying said second disc against the first disc;
a sealing disc placed coaxially against the second disc on the side opposite the first disc, the sealing disc being extended axially by a centering sleeve disposed between said drive shaft and the second disc, said second disc resting on the free end of the centering sleeve forming ball and socket joint and having the form of an annular portion of sphere, O-ring joints being placed between the sealing disc and the drive shaft.
The filtration device according to the present invention is particularly designed for the filtration of a liquid requiring handling precautions avoiding any contact with an operator, for example for a dangerous liquid, in particular in the nuclear industry.
To that end, the drum is preferably provided to be placed in an outer vessel recovering the liquid possibly overflowing from the drum via said opening into said vessel in the direction of a pipe for emptying the vessel. The vessel is preferably mounted in an outer glove box allowing access to the filtering elements with a view to replacement thereof. The filtration device preferably further comprises means for displacing the drum which, outside the periods of filtration, make it possible to position at least certain of said filtering elements opposite at least one glove of the glove box.
The filtration device according to the present invention is thus particularly suitable for reprocessing liquids and solid products issuing from the nuclear industry, the device being intended in particular for the filtration of a precipitate of plutonium oxalate in an acid solution, the geometry of the filter in that case being subcritical. In that case, the solution for washing the cake preferably includes nitric acid.